1. Field of the Invention
This invention relates generally to optical transmission systems and, more particularly, to polarization converters utilized in such optical transmission systems.
2. Description of the Related Art
Polarization rotators are building block elements in photonic integrated circuits (PICs). A polarization converter may be configured to accept an optical signal having a first polarization state at an input and convert the first polarization state into a second polarization state at an output. The first and second polarization states may be orthogonal. For example, the first polarization state may be a transverse electric (TE) polarization state and the second polarization state may be a transverse magnetic (TM) polarization state, the polarization converter converting the polarization state of the optical signal from the TE polarization state to the TM polarization state. Polarization converters are widely used in polarization diverse optical circuits, for example an optical circuit may be configured to accept optical signals of multiple polarization states, however process optical signals only with respect to one polarization state. A polarization converter, as part of the optical circuit for example, may then be used to convert the TM polarization state component of the optical signal into a TE polarization state component prior to processing. Such conversion of the polarization state affords the optical circuit designer the ability to use similar circuit elements, for example with similar designs, in the processing of the optical signal. After processing of the optical signal, one of the signals may be converted into an orthogonal polarization state by a second polarization rotator and then combined into a single output by a polarization beam combiner.
Polarization converters or rotators typically require precise geometry and tight control of critical dimensions of the rotator during fabrication. Turning to FIG. 1A, a waveguide 100 is fabricated into a polarization rotator. The waveguide 100 is typically fabricated by first providing various layers of semiconductor materials atop a top surface 110A of a substrate 110, and then etching back a top layer 112 to expose a notch 114, as shown in FIGS. 1B-1D for example. In one exemplary process, an etch mask 120A, shown in dashed line, is provided atop of the top layer 112, the etch mask 120A defining that area which will be etched or removed from the waveguide 100 structure to create the polarization rotator. The top layer 112 is then etched back a portion of the waveguide 100 width W to create the recessed portion or notch 114 giving the waveguide 100 an asymmetrical geometry along a longitudinal axis of the waveguide 100. FIGS. 1B-1D represent cross section views of the waveguide 100 at lines 1B, 1C and 1D, respectively. The cross section views as depicted in FIGS. 1B-1D are similar except for that portion of the top layer 112 which has been removed in an etching process. For example, with respect to FIG. 1B, at line 1B of FIG. 1A the width of the top layer 112 equals the width W of the waveguide 100 itself. FIG. 1C depicts the cross section at line 1C of FIG. 1A. As shown in FIG. 1C, in accordance with the etch mask 120A a portion of the top layer 112 has been removed to form the notch 114. FIG. 1D depicts the cross section at line 1D of FIG. 1A, the top layer 112 having a width of a about half the width W of the waveguide 100 forming notch 114.
Proper placement of the etch mask 120A is needed to provide the necessary asymmetric nature of the waveguide 100 in order to achieve the desired polarization rotation of an optical signal or electromagnetic wave propagating through the waveguide 100 along its length L. Waveguides are typically on the order of a few micrometers width and the ability to properly place the etch mask may be limited by corresponding stepper motors which move or otherwise position the etch mask upon the waveguide 100. For example, if the etch mask is laterally misaligned a distance, labeled Mask Offset, as compared to the desired position of the etch mask 120A, such as etch mask 120B shown in dash-dot line, the asymmetrical nature of the waveguide 100 is disturbed. Such misalignment of the etch mask 120B may result in undesirable results of the corresponding polarization rotator.
There is a need to develop a polarization rotator or converter, and corresponding processes, which are tolerant to lateral offsets of an etch mask, or equivalent structure, during fabrication.